Electrical filters find a wide variety of applications in analog circuits for selectively enhancing or suppressing certain frequencies. Low-pass filters are effective to pass low frequencies while suppressing high frequencies, while high-pass filters are adapted for passing high frequencies while suppressing lower frequencies. Notch-type filters function to suppress a very narrow band of frequencies, while passing frequencies on each side thereof. Equalization filters are effective to process groups of frequencies with different delays for each such group.
Filters can be constructed according to transfer functions which are representative of the noted low-pass, high-pass, notch or equalization characteristics. Filters have been conventionally constructed using discrete resistors, capacitors and inductors. Switched capacitor filters have more recently received favorable attention, in that such filters provide a practical approach for realizing precision filters in a monolithic form. Disclosed in U.S. Pat. No. 4,571,731 by Klinkovsky et al., is such a switched capacitor filter. The MOS LSI technology is particularly well adapted for the integration of switched capacitor filters. Essentially, a switched capacitor implementation of a filter is achieved by replacing each resistor critical to the transfer function with a capacitor and one or more switches, and connecting such an arrangement to an integrating amplifier. The capacitors can be constructed in an array using double-level polycrystalline techniques. The capacitors can then be connected together to form capacitances of desired value, and MOS switches can be utilized for switching such capacitors.
From the foregoing, a switched capacitor filter can be fabricated in a small wafer area by utilizing capacitors, transistor switches and high-performance operational amplifiers. Moreover, the transfer function of a switched capacitor filter depends only on the ratio of the various capacitors, and thus by changing the value of certain capacitors, different transfer functions can be obtained. Also required of a switched capacitor filter are a pair of clock signals which are biphase in nature, and nonoverlapping. As a result, certain capacitors can be charged and discharged so as to transfer a net charge during one clock period, and thereby define an average current flow likened to that flowing through a resistor. When the clock frequency is high with respect to the input analog signal frequency, the effects of sampling can be ignored, and thus the effective value of a resistor is proportional to the inverse of the frequency times the capacitance value.
A biquadratic switched capacitor filter can be implemented utilizing a two integrator loop topology, e.g., a pair of complementary integrators. A versatile biquadratic transfer function can take the form of EQU V.sub.O (z)/V.sub.i (z)=k(z.sup.2 +az+b).div.(z.sup.2 +cz+d).
Of course, the zeros of the transfer function are representative of the numerator of the equation, while the poles are represented by the denominator.
While switched capacitor filters have provided an excellent technique for implementing electrical filters using silicon integrated circuit technology, certain limitations hamper the optimal utilization thereof. For example, in cascading a number of biquadratic filter stages together, the time required for each such stage to settle is cumulative, thereby requiring a lower sampling rate, or a lower clock rate for switching the capacitors. As a practical matter, cascading 2-3 stages represents a present limitation. Also, there is currently no economical or practical technique for realizing negative capacitor values which can be encountered in the solution to various transfer functions. Hence, the full utilization of switched capacitor filters is often not realized.
An additional shortcoming of switched capacitor filters, in many instances, is the uneven capacitive loading of the biquadratic filter circuits during each clock phase. With such uneven capacitive loading, the bandwidth of each operational amplifier is different, or larger than required, thereby contributing to the noise factor of the filters. From the foregoing, it can be seen that a need exists for further improvements in biquadratic switched capacitor filters to more fully realize their capabilities.